Electric parking brake

ABSTRACT

An electric-parking-brake for a utility-vehicle, including: a feed-line for brake-pressure air; a discharge-line for brake-pressure air for a pneumatic-brake-device; a first-valve and a second-valve, each being switchable between a stable-state and an activated-state in response to electrical control-signals; and a valve-device which is connected between the feed-line and the discharge-line and exhibits a control-input, the valve device being switchable between a stable-state and an activated-state in response to control signals at the control-input, the feed-line being connected to the discharge-line in the activated-state, in which the first-valve in the stable-state or in the activated-state connects the control-input of the valve-device to the discharge-line, to retain a current-state of the valve-device when the brake-pressure air is applied to the discharge-line, and in the activated or stable state connects the control-input to the second-valve. Also described are an electric parking brake system, a utility vehicle, and a related method.

FIELD OF THE INVENTION

The present invention relates to an electric parking brake for a utilityvehicle and to a method for actuating an electric parking brake whichhas been provided, in particular, for a towing vehicle having anoptional trailer.

BACKGROUND INFORMATION

Parking-brake systems have to satisfy various safety requirements. Forinstance, it is required that they are capable of being operated stablyin at least two states. In a first state, moving the vehicle is to bepossible without a spontaneous activating of the parking brake beingtriggered—at least not so long as a sufficient pneumatic pressure isapplied. In the second state, a stable parked position is to bemaintained, even when the vehicle is not in operation.

Some parking-brake control systems for towing vehicles and trailers arefrequently based on an elaborate pneumatic piping system in the cabin ofthe utility vehicle. Such a system is shown, for instance, in FIG. 5 andincludes a first valve 510, a second valve 520, a shuttle valve 550 anda bypass valve 560, which are supplied with compressed air via a firststorage-pressure port 501 and a second storage-pressure port 502. Thetwo pneumatic valves 510, 520 are arranged, for example, in theinstrument panel of the towing vehicle. In the event of activation, thebrake-pressure air is routed to a parking-brake device (for example, aspring-type actuator in the towing vehicle and/or in the trailer),specifically via a vehicle parking-brake port 507 b (for the towingvehicle) and/or a trailer parking-brake port 507 a (for the trailer).The first storage-pressure port 501 and the second storage-pressure port502 are connected to a shuttle valve 550 (a so-called double-check valveor select-high valve) which makes a respectively greater pressure fromthe two storage-pressure ports 501, 502 available to a feed line 505leading to the first and second valves 510, 520.

The first valve 510 and the second valve 520 are each capable of beingoperated manually, independently of one another, by a driver. In theretracted state shown, the first valve 510 breaks the fluid connectionbetween the feed line 505 and the trailer parking-brake port 507 a. Inlike manner, the second valve 520 in the state shown breaks thepneumatic connection between the feed line 505 and the vehicleparking-brake port 507 b. The position shown of the first valve 510 andof the second valve 520 consequently represents the stable parkedposition. In this case, the spring-type actuators of the pneumaticbrakes have been vented by an existing pneumatic connection to a ventport 590, and the braking action is brought about by springs in thespring-type actuators. In the case of a storage pressure that is toolow, the two valves 510, 520 spring out by themselves and vent thespring-type actuators, so that the stable parked position is assumed.

If the towing vehicle are to be moved together with the trailer, thefirst valve 510 and the second valve 520 are pushed in manually by thedriver. Correspondingly, the first valve 510 opens the pneumaticconnection between the trailer parking-brake port 507 a and the feedline 505, and the second valve 520 opens the pneumatic connectionbetween the feed line 505 and the vehicle parking-brake port 507 b.

The bypass valve 560 has the following function. If the driver wouldlike to re-assume the parked position from the stable driving positionwhen both valves 510, 520 are located in the pushed-in state, and onlyretracts the second valve 520 from the driving position and manuallyholds the first valve 510 in the lower position, the bypass valve 560constitutes a pneumatic bypass to the vent port 590, in ordernevertheless to obtain the reduction of pressure in the system. As aconsequence, in every case a venting of the trailer parking-brake port507 a occurs, so that the trailer also assumes the stable parked state.

However, the parking-brake system shown requires a pneumatic pipingsystems in the cabin of the vehicle, which is elaborate. There istherefore a need to replace this conventional parking-brake controlsystem with more cost-effective and safe electronic solutions thatrequire less space in the instrument panel of the towing vehicle andnevertheless enable an automatic parking-brake function.

An example of an electronic parking-brake system is shown in FIGS. 6A,6B, wherein FIG. 6A represents the stable parked position and FIG. 6Brepresents the stable driving position.

The electric parking brake again includes a first valve 610, a secondvalve 620 and a valve device 630, which have been formed between a feedline 105 and a discharge line 107. The feed line 105, in turn, providesthe brake-pressure air, and the discharge line 107 (outlet) couples tothe spring-type actuator 660. In the stable parked position (see FIG.6A), by virtue of the spring the spring-type actuator 660 brings about abraking force which is exerted via the coupling element 670. In thedriving position (see FIG. 6B), the parking brake is released by thebrake-pressure air which moves a piston 662 contrary to the springforce.

The first valve 610 and the second valve 620 in this example areelectrically controllable solenoid valves which are magneticallycontrollable between two positions. The valve device 630 includes afirst inlet 631, a second inlet 632, a third inlet 633, an outlet 107and a vent 590. Connected between the first inlet 631 and the feed line105 is the first valve 610 which either supplies the first inlet 631with compressed air from the feed line 105 or vents it. Connectedbetween the second inlet 632 and the feed line 105 is the second valve620 which either supplies the second inlet 632 with compressed air fromthe feed line 105 or vents it. In the de-energized state, both valves610, 620 are in the vented position. The third inlet 633 is directlyconnected to the feed line 105.

The valve device 630 includes a spring 636 which biases a piston 635 inthe valve device 630 so that piston 635 closes the third inlet 633pneumatically via a closing element 639. This is the stable parkedposition in which, in addition, the first and second valves 610, 620vent the first and second inlets 631, 632.

The valve device 630 can be brought into the stable driving position viathe second inlet 632, by the second valve 620 admitting compressed airinto a chamber below piston 635 via the feed line 105. As a consequence,on an underside on piston 635 an additional pneumatic pressure isexerted which suffices to move piston 635 upward contrary to the springforce of the spring 636. Piston 635 can move upward until the closingelement 639 closes an upper opening having a cross-sectional area A1.This state is shown in FIG. 6B.

This state is similarly stable, since the force with which piston 635 ispushed upward is equal to the product of area A1 and the pressure P madeavailable by the compressed air, and area A1 has been chosen to belarger than area A2 which represents the cross-sectional area at thelower piston stop 635. Therefore piston 635 is held in the upperposition with a greater force. In order that this position is stillstable even when the second valve 620 is being vented, the spring forceF of the spring 636 has been chosen in such a way that:F<(A1−A2)*Pwhere (A1−A2)*P is the force with which piston 635 presses against thespring 636. Therefore the spring tension of the spring 636 cannot pushthe closing element 639 downward contrary to the compressive force.

If piston 635 is located in the upper position, as shown in FIG. 6B, atthe same time the outlet 107 is connected to the third inlet 633, havingthe consequence that the pneumatic compressed air is also applied to thespring-loaded cylinder 660 and moves piston 662 therein contrary to thespring tension of the spring-type actuator 660. Hence the brake isreleased and the vehicle can be moved. Since piston 635 remains stablyin the upper position, the vehicle can be moved even when the first andsecond valves 610, 620 are being vented (for example, are being switchedoff).

Disadvantages of the electronic variant of the parking brake that isshown are its complexity (the valve device 630 needs a total of fiveports) and the fact that the stable position depends on the springforce. If the spring force changes through aging processes, this mayhave the consequence that the valve device 630 can no longer be heldstably in the individual positions.

There is therefore a need for alternative solutions that, on the onehand, do not need any pneumatic piping in the cabins of the towingvehicle and, on the other hand, are capable of being realized withsimple pneumatic components.

SUMMARY OF THE INVENTION

At least some of the aforementioned problems are solved by an electricparking brake as described herein, by a utility vehicle as describedherein, and by a method for operating an electric parking brake asdescribed herein. The further descriptions herein define furtheradvantageous embodiments.

The present invention relates to an electric parking brake for a utilityvehicle, said parking brake including the following: a feed line forbrake-pressure air, a line for discharge of compressed air for apneumatic brake device, a first valve and a second valve, which are eachcapable of being switched between a stable state and an activated statein response to electrical control signals. In addition, the electricparking brake includes a valve device which is connected between thefeed line and the discharge line and exhibits a control input. The valvedevice is designed to switch between a stable state and an activatedstate in response to control signals at the control input, the feed linebeing connected to the discharge line in the activated state. In thestable state, the first valve connects the control input of the valvedevice to the discharge line, in order to retain a current state of thevalve device when the brake-pressure air is applied to the dischargeline. In the activated state, the first valve connects the control inputof the valve device to the second valve. The first valve consequentlyestablishes a feedback which constitutes a stable state so long assufficient pressure is present (in order to keep the valve device in theactivated state).

Within the scope of the present invention, signals may be bothelectrical signals and pneumatic signals. In addition, connections aregenerally intended to represent pneumatic connections, so that acomponent that is connected between two other components or connectedthereto is to be understood in such way that the circuit/connectionrelates, in particular, to a pneumatic flow.

Optionally, the second valve in the stable state connects the feed lineto the first valve, in order to activate the valve device (via the firstvalve), and in the activated state vents the connection to the firstvalve and to the control terminal of the valve device when the firstvalve is in the activated state. But the second valve can also connectthe feed line to the first valve in the activated state, in order toactivate the valve device, and in the stable state can vent theconnection to the first valve and to the control terminal of the valvedevice when the first valve is in the activated state. In both cases itis a question of a “reversal”, in the course of the state of the valvedevice can be altered.

Optionally, the first valve and/or the second valve are 3/2-way solenoidvalves which automatically assume the stable state in a de-energizedstate.

The valve device may be a booster valve with at least one spring-biasedpiston and with a vent opening. Optionally, the valve device is designedto vent the discharge line in the stable state, in the course of whichthe vent opening points in a direction of motion of the piston, or mayhave been arranged to the side thereof. For instance, the valve devicemay include the following: a housing with at least two chambers, a firstopening, a second opening and a third opening. The valve device may, inaddition, include the following: a first piston in the first chamber,which has been biased by a first spring element contrary to a pressureat the second opening, and a second piston in the second chamber, whichhas been biased by a second spring element, in order to interrupt aconnection between the first opening and the third opening in anunpressurized state. In the course of a supply of compressed air via thesecond opening, the first piston contacts the second piston, so thatboth pistons are displaced together, contrary to the spring tension ofthe first spring and the spring tension of the second spring, and theconnection between the first opening and the third opening isestablished. In the unpressurized state, the first piston moves awayfrom the second piston, as a result of which the third inlet isconnected to the vent opening.

Optionally, the parking brake includes a shuttle valve with a first portfor a first supply of compressed air and with a second port for a secondsupply of compressed air. The shuttle valve is designed to connect thefirst supply of compressed air or the second supply of compressed air tothe feed line, depending on where a higher brake pressure is applied.

The present invention also relates to an electric parking-brake systemfor a tractor (utility vehicle) with a parking-brake device, inparticular a spring-type actuator, and with a trailer parking-brakeport. The parking-brake system includes a first and a second electricparking brake as previously described. The first electric parking brakehas been formed between the compressed-air supply and a first dischargeline which couples to the braking device of the tractor. The secondelectric parking brake has been formed between the compressed-air supplyand a second discharge line which couples to the trailer parking-brakeport.

The present invention also relates to a utility vehicle with theparking-brake system and with a braking device that exhibits at leastone spring-loaded cylinder which in the unpressurized state activates aparking brake via a spring biasing force and releases the parking brakeby virtue of the brake-pressure air at the discharge line.

Optionally, the utility vehicle may exhibit a control device whichelectrically controls the first valve and/or the second valve, in order:

-   -   upon establishing a desire to drive off, to activate the valve        device by activating at least the first valve, and to ensure        that the second valve passes compressed air from the feed line;    -   upon establishing a desire to park, to bring the valve device        into the stable state by activating at least the first valve        into the stable state, and to ensure that the second valve vents        the connection to the first valve.

The present invention also relates to a method for actuating theelectric parking brake. The method comprises:

-   -   upon establishing a desire to drive off, activating the valve        device by activating at least the first valve;    -   upon establishing a desire to park, switching the valve device        into the stable state by venting the control input by activating        at least the first valve.

Optionally, the desire to park is triggered by at least one of thefollowing conditions:

-   -   an input by the driver,    -   reception of a corresponding signal of a control unit in the        case of autonomous vehicle operation,    -   establishing a standstill of the vehicle when least one        supplementary condition obtains which, in particular, includes        switching the ignition off or a continuous actuation of the        service brake or another condition.

Optionally, the desire to drive off is triggered by at least one of thefollowing conditions:

-   -   input by the driver,    -   actuating an accelerator pedal with the engine running,    -   reception of a corresponding signal of a control unit in the        case of autonomous vehicle operation,    -   actuating the service brake.

Optionally, the electric parking brake is part of the parking-brakesystem, and the actuating of the first electric parking brake and/or ofthe second electric parking brake can be undertaken individually orjointly or in succession.

This method, or at least parts thereof, may also have been implementedor stored in the form of instructions in software or on acomputer-program product, in which case stored instructions are capableof executing the steps according to the method when the method isrunning on a processor (for example, in a control unit of the vehicle).Therefore the present invention also relates to a computer-programproduct with software code (software instructions) stored thereon whichis designed to execute one of the previously described methods when thesoftware code is executed by a processing unit. The processing unit maybe any form of computer or control unit that exhibits an appropriatemicroprocessor that can execute a software code.

The exemplary embodiments of the present invention will be betterunderstood from the following detailed description and from theaccompanying drawings of the varying exemplary embodiments which,however, should not be understood in such a way that they restrict thedisclosure to the specific embodiments, but serve merely for explanationand comprehension.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electric parking brake according to an exemplaryembodiment of the present invention.

FIG. 2 shows an electric parking-brake system according to furtherexemplary embodiments.

FIGS. 3 and 3B illustrate further details of the electric parking-brakesystem according to further exemplary embodiments.

FIG. 4 shows a flowchart for a method for controlling an electricparking brake according to an exemplary embodiment.

FIG. 5 shows a conventional parking-brake control system with the aid ofpneumatic valves.

FIGS. 6A and 6B show an electric parking brake.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary embodiment of an electric parking brake for autility vehicle. The electric parking brake 100 includes a first valve110, a second valve 120 and a valve device 130 (for example, a boostervalve) which are pneumatically connected between a feed line 105 forbrake-pressure air and a discharge line 107 for the brake-pressure airleading to a pneumatic braking device. In this exemplary embodiment, thefirst valve 110 and the second valve 120 are 3/2-way valves which arecapable of being switched between a stable state and an activated statein response to electrical control signals.

The valve device 130 includes a first inlet 131, which is connected tothe feed line 105, a second inlet 132 and a third inlet 133 which isconnected to the discharge line 107. It will be understood that,depending on the directions of pneumatic flow, the inlets may also beoutlets. The second inlet 132 is a control input, in order to switchbetween a stable state and an activated state in response to (pneumatic)control signals. The control input 132 may also represent an inlet for apneumatic line, in order to move one or more pistons between twopositions or states with the aid of compressed air.

In addition, the valve device 130 shown includes a vent port 190 and abiasing device 136 which, for example, may include a spring. In thestable state, the third inlet 133 is connected to the vent port 190, sothat the discharge line 107 is automatically vented (for example, in theevent of a drop in pressure) and remains vented so long as no pneumaticcontrol signal is applied to the control input 132. In the activatedposition, the first inlet 131 is connected to the third inlet 133, andconsequently the feed line 105 is connected to the discharge line 107.

The 3/2-way valves 110, 120 also include three pneumatic ports and,additionally, respectively an electrical control terminal 112, 122. Thecontrol terminals are designed to switch the first 3/2-way valve 110and/or the second 3/2-way valve 120 between the stable state and theactivated state in response to (electrical) control signals. Forinstance, the two 3/2-way valves 110, 120 may be solenoid valves, inwhich case an electromagnet brings about the switching operations.

In the stable state, the first 3/2-way valve 110 connects, for instance,the discharge line 107 to the control input 132 of the valve device 130,in order to retain a current state of the valve device 130 in the caseof sufficient brake-pressure air. In the activated state, the first3/2-way valve 110 connects the control input 132 to the second 3/2-wayvalve 120.

In the stable state, the second 3/2-way valve 120 connects, forinstance, the feed line 105 to the first 3/2-way valve 110, in order toactivate the valve device 130 in the case of an activated first 3/2-wayvalve 110. In the activated state, the second 3/2-way valve 120interrupts a supply of compressed air to the first valve 110 and ventsthe connection to the first valve 110. If the first valve 110 has alsobeen activated, the valve device 130 is therefore brought into thestable state and the discharge line 107 is vented.

In further exemplary embodiments (not shown), the states of the secondvalve 120 (and similarly also of the first valve 110) can be chosenprecisely the other way round. Therefore the second valve 120 in thestable state can also vent the connection to the first valve 110, and inthe activated state can connect the feed line 105 to the first valve110.

FIG. 2 shows an electric parking-brake system for a utility vehicle (forexample, a towing vehicle) with a trailer parking-brake port 240, aspring-type actuator 140 and a shuttle valve 150 which is connected to afirst pressure port 101 and to a second pressure port 102. The shuttlevalve 150 is designed to transmit the respectively higher pressure fromthe first pressure port 101 and the second pressure port 102 to a feedline 105. The trailer parking-brake port 240 and the spring-typeactuator 140 are respectively connected to a discharge line 107 a, 107b, in order to be supplied with compressed air (in the driving state) orto be vented (in the parked state).

Correspondingly, two electric parking brakes 100A, 100B have been formedbetween the feed line 105 and the discharge lines 107 a, 107 b. A firstelectric parking brake 100A actuates the spring-type actuator 140 of thetractor and includes a first valve device 130 a which is controlled bytwo 3/2-way valves 110 a, 120 a, in order to supply the spring-typeactuator 140 with compressed air, or to vent it. A second electricparking brake 100B actuates a trailer parking brake via the trailerparking-brake port 240 and includes a second valve device 130 b which islikewise controlled by two 3/2-way valves 110 b, 120 b, in order tosupply the trailer parking-brake port 240 with compressed air or to ventit.

The two electric parking brakes 100A, 100B may have been constructed inthe same way and may function in the same way as the electric parkingbrake from FIG. 1. A renewed description is therefore unnecessary.

FIGS. 3A,B show further exemplary embodiments of the electricparking-brake system 100 from FIG. 2, wherein further details of thefirst and second valve devices 130 a, 130 b are represented. The firstand second valve devices 130 a, 130 b may have been constructedidentically and each include a first inlet 131, a second inlet 132, athird inlet 133 and a vent outlet 190. The valve devices 130 a, 130 beach include, in addition, a first piston 135 and a second piston 134,which have been biased correspondingly by a first spring 136 and asecond spring 137. The valve devices 130 a, 130 b shown have beenrepresented in a sectional representation, wherein the elements shownmay have been designed to be rotationally symmetrical, for instance.

According to one embodiment (see FIG. 3A), the vent outlet 190 has beenformed below the second piston 134 and therefore constitutes an openingin one of the directions of motion of the pistons. In a furtherembodiment (see FIG. 3B), the vent outlet 190 has been formed on a sidewall (laterally alongside pistons 134, 135), in order to open a chamberbelow the first piston 135 toward the outside.

When the second inlet 132 has been vented, the first piston 135 movesaway from the second piston 134, and the vent opening 190 is connectedto the third inlet 133. For this purpose, channels have been formed inpistons 134, 135, so that the compressed air is discharged from thethird inlet 133 either to the vent opening 190 on the underside (seeFIG. 3A) or to the vent opening 190 on the side wall (see FIG. 3B).

If compressed air is applied to the second inlet 132, the first piston135 is pressed down against the spring tension until the first piston135 comes into contact with the second piston 134 and then presses thelatter down contrary to the spring tension of the second spring 137.Since in this state the first piston 135 and the second piston 134 areconnected to one another, the vent opening 190 is closed and no longerin contact with the third inlet 133. Instead of this, the first inlet131 is connected to the third inlet 133, since the second piston 134opens the first inlet 131 in the course of being pressed down.

FIG. 4 shows a flowchart for a method for actuating the describedelectric parking brake. The method comprises:

-   -   upon establishing a desire to drive off, activating S110 the        valve device 130 by activating at least the first valve 110;    -   upon establishing a desire to park, switching S120 the valve        device 130 into the stable state by venting the control input        132 of the valve device 130 by activating at least the first        valve.

It will be understood that all the described functions of the electricparking brake 100 or of the parking-brake system can be executed asfurther optional steps.

This method may also be computer-implemented—that is to say, it may havebeen realized by instructions that have been stored on a storage mediumand are capable of executing the steps of the method when it is runningon a processor. The instructions typically comprise one or moreinstructions that may have been stored in varying ways on varying mediain, or peripheral to, a control unit (with a processor) and that, whenthey are read and are executed by the control unit, cause the controlunit to carry out functions, functionalities and operations that arenecessary for executing a method according to the present invention.

The software may, for instance, have been designed to switch on theparking brake of the towing vehicle and of the trailer individually orjointly, and opens it in accordance with the desire of the driver. Inaddition, the software can automatically engage the parking brake of thetowing vehicle and/or trailer at a standstill under certainsupplementary conditions. Similarly, under certain conditions thesoftware can automatically open the parking brake of the towing vehicleand/or trailer if, for instance, a desire of the driver to move off wasdetected.

Advantageous aspects of the electric parking-brake system may also besummarized as follows:

The first and second valve devices 130 a, 130 b are each, for instance,a double-seat booster valve for the parking brake of the towing vehicleor for the parking brake of the trailer, in which case each boostervalve 130 a, 130 b can be controlled with a first 3/2-way solenoid valve110 a, 110 b. This first 3/2-way valve 110 a, 110 b is capable of beingswitched between two switch positions: a “feedback” (in the stablestate) and a “reversal” (in the activated state). In the case of the“feedback”, the outlet 133 of the booster valve 130 a, 130 b isconnected to the control input 132 a, 132 b thereof, in order to sustainthe switching state thereof stably. In the case of the “reversal”switching position, the control input 132 a, 132 b of the booster valve130 a, 130 b is connected to a second 3/2-way solenoid valve 120 a, 120b.

The second 3/2-way solenoid valve 120 a, 120 b can now, in a firstswitching position, connect the control input 132 a, 132 b of therespective booster valve 130 a, 130 b to the atmosphere (venting), inorder to switch the booster valve 130 a, 130 b into the ventingposition, or, in a second switching position, connect to a storagepressure (from the feed line 105), in order to switch the booster 130 a,130 b into the venting position.

The first 3/2-way solenoid valve 110 a, 110 b is in the de-energizedstate (stable state) in the “feedback” switching position. After thereversal of the booster valve 130 a, 130 b, the first 3/2-way solenoidvalve 110 a, 110 b is firstly switched off again, so that it thereforeautomatically goes into the “feedback” switching position. After this,the switch position of the second 3/2-way solenoid valve 120 a, 120 b nolonger has any influence.

Both booster valves 130 a, 130 b and the 3/2-way solenoid valves 110 a,110 b thereof can, for instance, be supplied pneumatically from the twostorage circuits 101, 102 by a shuttle valve 150, as in conventionalparking-brake systems.

The respective booster piston 135 a, 135 b is pushed in the “venting”direction by a spring 136 a, 136 b. As a result, the booster valve 130a, 130 b can only be in the aerating position if it holds that:Storage pressure×Area of booster piston>Spring force.

If the storage pressure of both brake circuits 101, 102 falls below thispressure, the corresponding booster 130 a, 130 b will automatically gointo the venting position after overcoming the friction (on piston 135).Hence it is ensured that a vehicle that no longer has sufficient brakepressure for the service brake automatically comes to a halt with theparking brake.

The booster valve 130 a, 130 b may, in addition, be present in twovariants: with a central vent (see FIG. 3A) or with a side vent (seeFIG. 3B). The second 3/2-way solenoid valve 120 can be switched incurrentless manner to aerating.

Alternatively, it is likewise possible that the second 3/2-way solenoidvalve 120 is switched in currentless manner to venting.

With this parking-brake system and with a corresponding electroniccontrol system in an electronic control unit (ECO) (for controlling the3/2-way solenoid valves 110, 120) and also with two electricalpush-buttons as human/machine interface (HMI), all the functions such asare known from conventional parking-brake systems, and also diverseadditional automatic functions, can be carried out insoftware-controlled manner.

The features of the invention disclosed in the description, in theclaims and in the figures may be essential, both individually and inarbitrary combination, for the realization of the invention.

LIST OF REFERENCE SYMBOLS

100A, 100B electric parking-brake units

101, 102, 501, 502 compressed-air supply

105 feed line

107 discharge line

110, 120, 610, 620 first and second 3/2-way valves

111, 113, 114 ports of the first 3/2-way valve

121, 123, 124 ports of the second 3/2-way valve

112, 122 electrical control terminals

116, 136 biasing device

130, 630 valve device

131, 133, 631, 632, . . . inlets

132 control input

134, 135 piston of the valve device

137, 636 spring

140, 660 spring-type actuator

150, 550 shuttle valve

190, 590 vent

240 brake port for trailer

505 feed line

507 a,b parking-brake ports

510, 520 manual valves

560 bypass unit

639 piston element

662 spring-loaded piston

670 brake component

A1, A2 effective pressure areas

The invention claimed is:
 1. An electric parking brake for a utilityvehicle, which includes a tractor or towing vehicle, comprising: a feedline for brake-pressure air; a discharge line for brake-pressure air fora pneumatic brake device; a first valve and a second valve which areeach switchable between a stable state and an activated state inresponse to electrical control signals; and a valve device, which isconnected between the feed line and the discharge line and exhibits acontrol input, the valve device being switchable between a stable stateand an activated state in response to control signals at the controlinput, the feed line being connected to the discharge line in theactivated state; wherein the first valve in the stable state or in theactivated state connects the control input of the valve device to thedischarge line, to retain a current state of the valve device when thebrake-pressure air is applied to the discharge line, and in theactivated state or in the stable state connects the control input to thesecond valve, wherein the valve device includes a first inlet, which isconnected to the feed line, a second inlet and a third inlet which isconnected to the discharge line, wherein the second inlet is a controlinput for switching between the stable state and the activated state inresponse to the control signals, and wherein the control inputrepresents an inlet for a pneumatic line to move one or more pistonsbetween two positions or states with compressed air, wherein the firstvalve, the second valve and the valve device are pneumatically connectedbetween the feed line for brake-pressure air and the discharge line forthe brake-pressure air leading to a pneumatic braking device, whereinthe first valve and/or the second valve are 3/2-way solenoid valveswhich automatically assume the stable state in a de-energized state, andwherein the 3/2-way solenoid valves are switchable between a stablestate and an activated state in response to electrical control signals,and wherein the first valve establishes a feedback which constitutes astable state so long as sufficient pressure is present to keep the valvedevice in the activated state.
 2. The parking brake of claim 1, whereinthe second valve in the stable state connects the feed line to the firstvalve, to activate the valve device, and in the activated state vents aconnection to the first valve and, when the first valve is in theactivated state, vents a connection to a control terminal of the valvedevice.
 3. The parking brake of claim 1, wherein the second valve in theactivated state connects the feed line to the first valve, to activatethe valve device, and in the stable state vents a connection to thefirst valve and, when the first valve is in the activated state, vents aconnection to a control terminal of the valve device.
 4. The parkingbrake of claim 1, wherein in the stable state, the first 3/2-way valveconnects the discharge line to the control input of the valve device toretain a current state of the valve device in the case of sufficientbrake-pressure air, and in the activated state, the first 3/2-way valveconnects the control input to the second 3/2-way valve, and wherein inthe stable state, the second 3/2-way valve connects the feed line to thefirst 3/2-way valve to activate the valve device for an activated first3/2-way valve, and in the activated state, the second 3/2-way valveinterrupts a supply of compressed air to the first valve and vents theconnection to the first valve, and wherein when the first valve has alsobeen activated, the valve device is brought into the stable state andthe discharge line is vented.